1,720,984 research outputs found
Energetics of transient enhanced diffusion of boron in Ge and SiGe
No full textWe study the energetics and migration of boron in Ge and ordered, Si-epitaxial Si0.5Ge0.5 via first-principles calculations, considering specifically the interstitial-mediated mechanism previously associated with B transient enhanced diffusion in Si. The temperature dependence of the migration length lambda of a B-interstitial complex is calculated from migration barriers and dissociation energies. In Ge, the migration length hardly depends on temperature, while in SiGe it is similar to that in Si, due to the preference of B for Si-like equilibrium sites and diffusion paths. The calculated solubility of B in Ge is similar to that in Si, about 1x10(19) cm(-3). In Si-epi strained SiGe the solubility is instead enhanced by two orders of magnitude, and in free-standing SiGe by one order of magnitude. The calculated activation energy for B diffusion in Ge (similar to4.5 eV) is considerably higher than in Si (similar to3.6 eV) and in our model SiGe, in accordance with recent experiments
Methylammonium fragmentation in amines as source of localized trap levels and the healing role of Cl in hybrid lead-iodide perovskites
Full text linkThe resilience to deep traps and localized defect formation is one of the important aspects that qualify a material as a suited photoabsorber in solar cell devices. Here we investigate by ab initio calculations the fundamental physics and chemistry of a number of possible localized defects in hybrid methylammonium lead-iodide perovskites. Our analysis encompasses a number of possible molecular fragments deriving from the dissociation of methylammonium. In particular, we found that in stoichiometric conditions both ammonia and methylamine molecules present lone-pair localized levels well within the perovskite band gap, while the radical cation CH2NH3+ observed by EPR after irradiation injects partially-occupied levels into the band gap but only in p-type conditions. These defects are thus potentially capable of significantly altering absorption and recombination properties. Amazingly, we found that additional interstitial Cl is capable of removing these localized states from the band gap. These results are consistent with the observed improvement of photoabsorption properties due to the Cl inclusion in the solution processing
Dielectric properties of two phases of crystalline lutetium oxide
No full textHigh-dielectric constant oxides are the focus of intense current research. As a contribution to the rationalization of the search for candidate materials, we compare the dielectric properties obtained from first-principles linear-response calculations for two phases-the ground state bixbyite and the competing hexagonal structures-of the crystalline oxide Lu2O3. The dielectric constants of bixbyite is about 12 and that of hexagonal is about 19 (the electronic constant being about 4.5 in both cases). This difference is due almost exclusively to the vibrational properties, with minor or no influence of charge anomaly differences; as these are related mainly to oxygen vibrations, we argue that the dielectric properties of sesquioxides will be determined mostly by their preferred structure, i.e. they will be poorer for bixbyite (e.g. Y2O3) than for hexagonal (e.g. La2O3) sesquioxides. (c) 2004 Elsevier Ltd. All rights reserved
Dielectric properties of rare-earth oxides: General trends from theory
No full textWe present a theoretical perspective on general aspects of the dielectric response and dynamical properties of some rare-earth oxide systems. We deal in particular with sesquioxides and aluminates, the latter both in the amorphous and crystalline phases
Dielectric properties and long-wavelength optical modes of the high-kappa oxide LaAlO3
No full textWe present ab initio density-functional and self-interaction-corrected calculations on the structure, long-wavelength vibrational modes, and static dielectric properties of the crystalline high-kappa distorted-perovskite oxide LaAlO3. The orientationally averaged dielectric constant kappa similar or equal to 23.3 is in good agreement with experiment, and its value is dominated by the ionic response. We find it important to calculate the dynamical charges by a self-interaction-corrected scheme, bringing about a similar to 15% improvement on the DFT result. We report on the infrared- and Raman-active vibrational modes, which are in good agreement with experiment where applicable, and we discuss the origin of the recently observed dielectric anisotropy
Reorientable dipolar Cu-Ca antisite and anomalous screening in CaCu3Ti4O12
No full textBased on first-principles calculations, we show that the abundant Cu-Ca antisite defect contributes sizably to dielectric screening in single-crystal CaCu3Ti4O12. Cu-Ca has a multi-minimum off-center equilibrium configuration, whereby it possesses a large and easily reorientable dipole moment. The low-temperature and frequency cut-off behavior of Cu-Ca-induced response is consistent with experiment
Radiative recombination and photoconversion of methylammonium lead iodide perovskite by first principles: properties of an inorganic semiconductor within a hybrid body
No full textThe excellent photoconversion properties of lead iodide hybrid perovskites, used as absorber in solar cell devices with power conversion efficiencies exceeding 15%, are explained on the basis of ab initio calculated radiative recombination rates and minority carrier lifetimes. Brad ∼ (0.5-1.5) × 10-9 s-1 cm3 and minority lifetime ∼103 ns were obtained for a doping concentration of n ∼ 1015 cm-3 at room temperature. These values, comparable to those of typical optoelectronic semiconductors (e.g., GaAs), reflect the very nature of the perovskite: fully solution-processable owing to its hybrid nature, and yet a truly inorganic semiconductor for with regard to photoconversion properties. Recombination rates are also used to quantitatively describe the maximum limit of power conversion efficiency potentially achievable by these systems, for example, 21% for a 200 nm thick perovskite film and 23% for a 300 nm thickness
Dielectric and vibrational properties of bixbyite sesquioxides
No full textMost sesquioxides (X(2)O(3)) with a rare-earth cation crystallize in the cubic structure known as bixbyite, also adopted by oxides with smaller trivalent cations (e.g., Sc and Y). Here we discuss the dielectric behavior of these crystals on the basis of results of density-functional calculations for a selection of them, namely, Sc, La, Dy, and Lu binary sesquioxides. Vibrational modes can be categorized in two groups: one mostly localized on cations and with low frequencies and another with higher energies localized on oxygens. In rare-earth oxides the frequency of cation-localized modes shows a weak dependence on cation type; oxygen modes, instead, increase their energy as the cation radius decreases from La to Lu. In the case of the smaller Sc, frequencies increase for all modes and the smaller cation/oxygen mass ratio allows for a significant mixing of oxygen and cation displacements at intermediate frequencies. In the final analysis, we conclude that bixbyites all have relatively small dielectric constants and rationalize the reasons thereof
Direct imaging of the core-shell effect in positive temperature coefficient of resistance-BaTiO(3) ceramics RID A-3737-2012 RID C-5307-2009
No full text- …
